How do you explain Hebbian Learning in an intuitive way?
Hebbian learning is trying to answer "How the strength of the synapse between 2 neurons evolve over period of time based on the activity of the 2 neurons involved.". You can call it learning if you think learning is just strengthening of synapses. 
The connection between 2 neurons are called synapse. A synapse is the point where the axons of a neuron meets with the dendrites of another neuron.   
The 2 neurons are connected to each other but they are also connected to other neurons as well. So it may happen that both the neurons gets activated by their other connected neurons at the same time.
The elements refer the 2 neurons. The equation is basically saying that the synapse strength between i and j neuron at time (n+1) depends on its strength at time n plus activations of the i and j neurons at time n.